Get a grade for your twitter profile. I was rated 1m out of 8m, whatever that means. Used by pay4tweet.com to gauge price of paid tweets.
An interesting business focused app for twitter. Handle twitter content in a biz friendly fashion, multiple accounts, one login, see threads, set tasks to follow up, etc. Looks useful.
Summize.com is a live search engine for twitter. You can grab a feed of queries, etc. Very simple, very light, very cool.
TweetScan is a Twitter search engine. You can watch all public tweets and scan then for keywords. Very handy when getting the low down on something (or people not following hashtags).